The present invention relates to the positioning of the read/write transducer heads of an hard disk (HD) in a designated landing zone when requested or when the electrical power is removed from the drive. In particularly it relates to the detection of the back electromotive force (BEMF) of the motor involved in the positioning of the read/write transducer heads.
A recent parking system (Ramp Loading technology) automatically performs a park when HD driver power supply fails or when the HD controller asks for it, by means of a Voice Coil Motor (VCM).
To obtain a ramp loading system, it is mandatory to have a signal at least proportional to the speed of the motor, in order to have a good control of the positioning of the read/write transducer heads.
In fact the BEMF measurement is compared to a velocity command signal in order to sense deviation of the actual motor speed from the desired speed, and in response adjusts the drive applied to the motor to correct for the speed deviation.
Since no servo tracks are available on the ramp, VCM speed is not known. This is the reason why information about speed have to be obtained by the motor itself.
In fact back electromotive force is proportional to VCM speed through:                     E        =                              Ke            ·            ω                    =                                    Ke              armlenght                        ·            speed                                              [        1        ]            
where Ke is the proportionality coefficient between angular speed and the back electromotive force.
Nowadays, two possible systems are known in order to obtain the speed detection.
A first way of sensing the BEMF is to use the voltage across the power bridge, that is the driver of the VCM, and the current flowing in the motor to compute the BEMF generated by the motor (continuous mode).
The second approach considers that if the Voice Coil power bridge is put in a tristate condition and the time for a complete current decay in the motor is elapsed, no current is present in the VCM and then the only voltage read across the coil is the back electromotive force (discontinuous mode).
Ramp Loading systems working in continuous mode suppose that the BEMF of the VCM is read continuously in time and it is not sampled.
In reality, the BEMF measured across a motor coil is not perfectly proportional to the motor rotational speed. Factors responsible for the imperfection are the motor resistance Rm, the sense resistor Rs and the elements (resistance and amplifiers) used in the measurement circuit.
The measured BEMF, then, can be viewed as the sum of these error components and an ideal BEMF to which the motor rotational speed is proportional.
In some applications, however, it is desirable to more accurately control motor speed. In such applications the BEMF measurement error is unacceptable. One example is the case mentioned of a voice-coil motor for a head actuator. It is important to accurately control the speed of a read/write head as it is being loaded onto a disk, so that the head does not strike the disk hard and cause damage. Similarly, it is important to avoid striking the head against a head stop when retracting the head from the disk.
Known circuits are able to accurately measure the BEMF of a VCM but require a double calibration circuit to reduce said measurement error.
In view of the state of the art described, it is an object of the present invention to provide a circuit able to accurately measure the BEMF of a VCM with a single calibration circuit.
According to the present invention, these and other objects are attained by means of a BEMF detection circuit for a voice-coil motor operative to continually generate a signal proportionally to the velocity of said voice-coil motor comprising a algebraic summing node producing at its output the BEMF of the voice-coil motor and receiving: a first voltage proportional to the voltage across the voice-coil motor; a second voltage representing the product of a first multiplier factor and a voltage proportional to the current in the coil; a third voltage representing the product of a prefixed bias voltage Vref and a second multiplier factor; said third voltage is calibrated by a single calibration circuitry operative to calibrate said second multiplier factor in response to a calibration control signal, in order to cancel said second voltage.
Such objects are also attained by a BEMF detection circuit for a voice-coil motor operative to continually generate a signal proportionally to velocity of said voice-coil motor such that said signal is the sum of a first signal component, a second signal component and a third signal component; the first signal component representing the product of a first multiplier factor and the voltage across the coil, the second signal component representing the product of a second multiplier factor and the current in the coil; the third signal component representing a signal able to eliminate said second signal component.
Thanks to the present invention, it is possible to provide a circuit able to accurately measure the BEMF of a VCM which is more precise, require less circuits and therefore less space.